76 



Sirocco, or distant origin of this wind, is not a tenable 

 theory, but that it has a local origin. To explain shortly 

 the present state of knowledge with regard to this important 

 climate factor ! If air rises or comes into less barometrical 

 pressure it must expand, and this expansion requires heat, 

 which is taken from the air brought to that spot, and thus 

 the temperature falls. Should on the other hand the air 

 descend or come into greater barometrical pressure the 

 contrary effect takes place, and the temperature rises. If 

 the air be quite dry the difference of temperature thus 

 caused is 1° Cent, for each 101 metres that the air has risen 

 or fallen. The air, however, is never quite dry, and there- 

 fore the calculation is not quite as simple as this, for when 

 air containing moisture rises it is lowered in temperature, 

 and so gradually reaches its dew point, when some of the 

 moisture will be precipitated as snow or rain, and in this 

 way the latent heat is set free, so that the fall of temperature 

 of air containing moisture is less than that of dry air rising 

 to the same height. In descending the temperature of the 

 air rises, but the amount of absolute moisture remains the 

 same, so tliat the air is constantly becoming relatively drier, 

 and it will thus be seen that the rise of temperature when 

 the air descends is more rapid than the fall when the air 

 ascended. Thus air with a temperature of 20° Cent, and 86 

 per cent of relative moisture would in passing over a 

 mountain 2500 metres higher deposit a part of its moisture, 

 and coming down to the level from which it started would 

 on the other side of the mountain have a temperature of 

 80'5 cent, and relative moisture 29 per cent.* 



I wish, as far as possible, to avoid in any way entering 

 into the merits of the rival places of Davos and St. Moritz, 

 but in consequence of the different configuration, the fohn 

 affects the two places so differently that in St. Moritz we 

 hardly ever hear anyone speaking of it, whereas in Davos 



• For fuller explanation see Mohn " Meteorologie," 2ncl ed., p. 174. 



